Got black screen when starting simulator

I'm using Xcode v6.1 on Mac OS X Mavericks v10.9.5. I'm new to iOS so i'm developing a simple app for learning. It's using two frameworks such as UI Kit and Foundation. When running my app in simulator, sometimes it worked but some other times i just showed black screen as shown in the image below.

As you can see on the title bar of the simulator, the app was built and run on iPhone 4s. Then I changed the destination device to iPhone 5 or newer, it worked. I don't know why but perhaps my application is using some classes or libraries which are not supported by iPhone 4s.

To change the running device, click on Product menu then Destination and select iPhone 5. After that, clean the project (command+shift+K), and build and run (command+R) it again.